In existing wireless technologies, signal repeating devices and systems, such as repeaters or distributed antenna systems (“DAS”), are used to extend the coverage of an overall wireless system beyond the range of traditional base stations. For example, an overall cellular or wireless communication system may consist of a plurality of base transceiver stations (“BTS”) or base stations that communicate with each other and with mobile user devices, such as cellular mobile phones, to provide a defined coverage area. In such coverage areas, there are often smaller geographical areas that have very low signal coverage, as provided by one or more of the base stations. For example, such areas of low signal coverage may be within buildings or in areas that are otherwise obstructed, such as by terrain features or man-made structures. Rather than simply implementing another costly and large base station in the area to provide coverage in such low signal areas, repeaters and distributed antenna systems are often utilized.
The distributed antenna system receives downlink signals from one or more donor base stations and distributes those signals via cable, such as fiber optics, coaxial cables, or copper twisted pair cable, throughout a building or other coverage area. At designated access points in the system, remote units amplify the downlink signals and transmit them to user equipment, such as mobile phones over radiating antennas. At those same access points, uplink signals are received by the remote units from mobile users, and the uplink signals are amplified, filtered and sent back through the distributed antenna system, where they are summed together and transmitted to the donor base station.
Many challenges exist with contemporary distributed antenna systems, one of which is maintaining a suitable noise floor in the system. Summing of the uplink signals from the various multiple remote units cumulatively adds the noise floors of each of the remote units. For example, a large system with 100 remote units summed together may raise the noise floor approximately 20 dB relative to the noise floor of a single remote unit. In order to compensate for the rise in the noise floor of the system, the mobile units interacting through the system must increase their transmit power. Based on the above example of a 20 dB rise in the noise floor, the mobile units will therefore need to increase their power by 20 dB in order to maintain a minimum acceptable carrier to noise ratio (C/N) coming out of the distributed antenna system. Since mobile units have a limited amount of transmit power that they can generate, this required increase in transmit power for the repeating system ultimately decreases range of the mobiles or the maximum distance from the remote unit of the distributed antenna system that the mobile unit can effectively communicate.
An additional challenge caused by the large noise floor rise is base station desensitization. Depending on path loss to the base station from the remote system, the noise floor contributed from the distributed antenna system could arrive at the base station at a higher level than the base station's own noise floor. When this occurs, the base station will be desensitized, potentially resulting in a shrinking coverage area for the base station. To assist in preventing the loss of coverage area, the uplink gain of the distributed antenna system should be set to a fairly low value to assure that the received noise floor from the distributed antenna system arrives at a level below the base station's own noise floor. For example, the distributed antenna system gain might be set such that the distributed antenna system noise floor arrives at the base station approximately 10 dB below the base stations own noise floor. In this situation, adding the distributed antenna system noise to the base station noise will result in only about a 0.4 dB rise in the overall floor noise at the base station receiver.
From the perspective of the mobile unit in the distributed antenna system having 100 remotes units, a 20 dB rise in the noise floor, as described above, requires the mobile units to increase their transmit power by 20 dB. The mobile units must also increase their power another 10.4 dB to overcome the base station noise floor which is 10.4 dB above the distributed antenna system noise floor. This further reduces the distance from the respective remote unit that mobile units can communicate.
To improve this situation, a squelch or muting can be applied to the uplink of any remote unit that has no traffic. This generally assists in decreasing the distributed antenna system noise rise and, for the example above, may also assist in decreasing the 0.4 dB noise rise at the base station. However, the mobile units can only decrease their transmit power a few tenths of a dB (0.4 dB max for the above example) in order to maintain the same C/N ratio at the base station. Therefore, while muting one or more remotes do assist in reducing base station desensitization slightly, it does not allow the mobile unit to significantly decrease its transmit power. Therefore, such a feature does not address the problems noted above or significantly increase the range of a distributed antenna system remote unit.
Embodiments of the present invention address these and other challenges in the prior art as discussed further below, and provide a significant advantage over contemporary distributed antenna systems having a large number of remote units.